Method for starting a submerged arc welding process and welding apparatus

ABSTRACT

A method comprising an arc ignition phase (IP), an arc-stabilizing phase (AP) and a stable arc phase (SP). The arc stabilizing phase comprises an initial sub-phase (IS) comprising the step of feeding at least one hot wire (4, 12) at constant feed speed and a main sub-phase (MS) comprising the steps of feeding said hot wire at constant feed speed and feeding at least one cold wire (22) at constant feed speed. The stable arc phase comprises the steps of continuously adjusting the feed speed of the hot wire and continuously adjusting the feed speed of the cold wire. The invention also relates to a welding apparatus (1) for carrying out the method. The welding apparatus comprises a hot wire feeding means (150), a contact means (2), a cold wire feeding means (35) and a control unit (31). The control unit is adapted to control said hot wire feeding means to feed the hot wire at a constant feed speed during the initial sub-phase, feed the hot wire at a constant feed speed during the main sub-phase and to continuously during the stable arc phase adjust the feed speed of the hot wire. The control unit is adapted to control said cold wire feeding means to feed the cold wire at a constant feed speed during the main sub-phase and continuously during the stable arc phase adjust the cold wire feed speed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 14/440,091, filed on May 1, 2015, and titled“Method for Starting a Submerged Arc Welding Process and WeldingApparatus”, which is a national stage entry of PCT Application No.PCT/EP2012/071685, filed on Nov. 2, 2012, and titled “Method forStarting a Submerged Arc Welding Process and Welding Apparatus”. Thedisclosures of the above applications are incorporated herein byreference in their entireties.

TECHNICAL FIELD

The invention relates to a method for starting a submerged arc weldingprocess. The method comprises an arc ignition phase, an arc-stabilizingphase and a stable arc phase.

The invention also relates to a welding apparatus for carrying out themethod according to the invention. The welding apparatus comprises hotwire feeding means for feeding at least one hot wire towards a workpiece, contact means for transferring current to said hot wire for arcgeneration, a cold wire feeding means for feeding at least one cold wiretowards the work piece, and a control unit adapted to control said hotand cold wire feeding means during an arc ignition phase, anarc-stabilizing phase and a stable arc phase.

BACKGROUND OF THE INVENTION

It is known to use a consumable electrode to conduct a weld currentthrough a work piece. The weld current forms an arc between theconsumable electrode and the work piece to create a weld pool on thework piece. A consumable electrode of this type is throughout thisspecification referred to as a hot wire.

Submerged arc welding (SAW) is a welding method characterized by highproductivity and quality, often used for longer welding seems in thickermaterials. Submerged arc welding is characterized in that the meltedmaterial and the arcs are protected beneath a layer of pulverized flux.The flux melts in part during the process, thus creating a protectinglayer of slag on the weld pool.

To achieve the highest productivity possible with submerged arc welding,one strives for increased weld speed and the highest possible depositionrate, i.e. the rate at which weld metal is deposited onto the work piecesurface. At the same time, the heat input should be kept on a level thatpreserves the mechanical properties of the welded parent material andthe weld should have mechanical properties of a certain level.

One way to increase the deposition rate is to use a plurality of hotwires to create a single weld puddle. Usually 2-3 hot wires are used,however, usage of up to 6 hot wires is known. Using more than one hotwire in a single weld puddle increases the deposition rate and thereforeimproves the economy of welding. It also enables improved weld qualitydue to the possibility of assigning leading and trailing hot wires withdifferent tasks.

Another way to improve the deposition rate is to add one or moreelectrodes that melt without formation of arcs. These electrodes arethroughout this specification referred to as cold wires. A cold wire isfed towards a molten weld pool in close proximity to one or more hotwires. The cold wire is melted by resistance heating as well as by theheat generated by the hot wires.

The introduction of cold wire material into the weld puddle may lead toimproved control of the composition of the weld alloy, which may lead toimproved welds. It is preferable to introduce the cold wire in thevicinity of and preferably into an arc generated by a hot wire (evenmore preferably in the vicinity of or into arcs generated by a pluralityof hot wires). A cold wire increases the deposition rate withoutincreasing the heat input. Feeding cold wire material into the weldpuddle may lead to an increase of productivity of up to 100% withoptimized welding parameters.

It is desirable to produce a stable arc as quickly as possible at thebeginning of a welding process. The presence of unstable arcs at thebeginning of the welding process may cause weld defects such asinclusions, splatter, and poor mechanical properties in the weldedobject. The presence of unstable arcs may also lead to a reduced meltingrate and, as a consequence thereof, the hot wire may hit the bottom ofthe weld puddle.

Likely consequences are bending of the hot wire and displacement of thewelding head. An unstable welding start may also cause arcs to protrudeout of the flux cover, which can damage the eyes of the welder.

Unstable arcs are a common problem when cold wires are used in thewelding process, as cold wires tend to increase the welding processinstability. Insufficient melting of a cold wire may cause it to strikethe parent material through the melt pool. This may cause weld defectsand inclusions in the weld metal of unmelted cold wire material as wellas buckling of the cold wire and swaying of the welding equipment.

Delayed establishment of a stable arc is a particularly common problemin welding processes involving a plurality of hot wires, wherein manyarcs strike at the same time. This is particularly the case in twinsetups, wherein two hot wires are connected to a common power source.Moreover, the inclusion of a cold wire between two hot wires increasesthe distance between said hot wires, which makes it even more difficultfor the hot wires to produce stable arcs.

US 2006/0016792 A1 addresses the issue of stable arc generation. Awelding wire is delivered to a weld area at a run-in speed. Afterdetection of an initial arc the wire feed speed is adjusted to a minimumvalue for a predetermined period of time (e.g. approximately 50 ms).Thereafter, the wire feed speed is set to a relatively stable feed speedfor welding.

The solution proposed in US 2006/0016792 A1 is complex, does not solvethe problem of the destabilizing effect of the cold wires and does notrelate to SAW.

A first object of the invention is to provide a method for starting asubmerged arc welding process including one or more cold wires, whichmethod ensures high weld quality already at the beginning of the weldingprocess.

A second object of the invention is to provide a welding apparatus forcarrying out said method.

BRIEF DESCRIPTION OF THE INVENTION

A phase preceding another phase does not have to be immediatelypreceding that phase. Other phases may be executed between said phases.A phase subsequent to or following another phase does not have to beimmediately following that phase. Other phases may be executed betweensaid phases. A phase may comprise any number of sub-phases.

Throughout this specification, references are made to weldingparameters. A welding parameter is a parameter that has a directinfluence on the welding process. Examples of welding parameters arewelding current, arc voltage, welding speed, hot wire feed speed andcold wire feed speed.

Welding parameters related to or dependent on one another are weldingparameters that, directly or indirectly, influence one another. Awelding parameter determined in dependence on another welding parametercan be determined from a measured value of said other welding parameteror from a measured value of a welding parameter related to said otherwelding parameter.

An active welding parameter is a welding parameter that is manually orautomatically adjusted during welding in response to changes in weldingconditions. Active welding parameters can be adjusted, directly orindirectly through adjustment of other active welding parameters, tomaintain one or more non-active welding parameters at an essentiallyconstant level.

Welding conditions are factors that influence the welding process.Examples of welding conditions are the shape and material properties ofthe work piece.

The first object of the invention is achieved with a method as definedin independent claim 1.

The method for starting a submerged arc welding process according to theinvention comprises, in the following order, an arc ignition phase,comprising the step of producing an arc, an arc-stabilizing phase and asubsequent stable arc phase. The arc-stabilizing phase comprises atleast one initial sub-phase and a least one main sub-phase. The initialsub-phase comprises the step of feeding at least one hot wire towards awork piece at a constant feed speed. The main sub-phase comprises thesteps of feeding said hot wire towards said work piece at a constantfeed speed and feeding at least one cold wire towards said work piece ata constant feed speed. The stable arc phase comprises the steps ofcontinuously adjusting the feed speed of the hot wire in dependence onat least a welding current transferred through said hot wire andcontinuously adjusting the feed speed of the cold wire in dependence onat least one hot wire feed speed of the stable arc phase.

An arc is ignited during the arc ignition phase. The ignition phase endsand the arc-stabilizing phase begins upon detection of said arc. The arcis stabilized during the arc-stabilizing phase. The arc-stabilizingphase may have a predetermined length or may be automatically terminatedwhen a stable arc has been detected. The arc-stabilizing phase may alsobe manually terminated when a user of a welding apparatus detects astable arc or when the welding apparatus informs said user that a stablearc has been detected. The end of the arc-stabilizing phase marks thebeginning of the stable arc phase. A stable arc is now present betweenthe hot wire and the work piece and welding can be carried out withoptimal results.

A welding start can also be divided into an arc ignition phase(identical to the ignition phase preceding the arc-stabilizing phase)and a welding phase, in some embodiments separated by one or morestart-up phases. Welding on the work piece is carried out mainly duringthe welding phase. In embodiments wherein the welding phase followsimmediately after the ignition phase, the arc-stabilizing phase and thewelding phase begins simultaneously. That is, welding is carried outalmost from the beginning of the welding process, even before a stablearc has been generated. In embodiments wherein the ignition phase andthe welding phase are separated by one or more start-up phases, thearc-stabilizing phase begins simultaneously with the first start-upphase.

The start-up phases are included to increase the likelihood of a stablearc being present when the welding phase begins (welding during thestart-up phases may be carried out on a run on/run off plate). In theseembodiments, the arc-stabilizing phase may end before, simultaneouslywith or after the beginning of the welding phase. Each sub-phase of thearc-stabilizing phase, with the possible exception of the lastsub-phase, begins and ends simultaneously with a corresponding start-upphase. The arc-stabilizing phase may have the same length as thecombined length of the start-up phases, in which case the number ofsub-phases is equal to the number of start-up phases. However, thearc-stabilizing phase may also extend into the subsequent welding phase,in which case there may be an additional, final sub-phase covering theperiod between the end of the last start-up phase and the beginning ofthe welding phase. It is also possible to extend the last main sub-phaseto the beginning of the welding phase. The arc-stabilizing phase mayalso end before the end of the last start-up phase, in which case thelast main sub-phase ends before the end of the last start-up phase.

The welding process carried out during the stable arc phase is aconstant amperage (CA) welding process. A CA welding process is aprocess wherein an arc voltage level remains constant and the amperagelevel of the welding current transferred through the hot wire ismaintained at a given level through adjustment of the hot wire feedspeed. The amperage level is related to the distance between the hotwire end and the work piece. A hot wire feed speed increase will resultin a welding current increase and a hot wire feed speed reduction willresult in a welding current reduction. Disturbances, such as stick outvariations caused by work piece surface irregularities, variations inthe welding process or joint configurations, sometimes change thedistance between the hot wire end and the work piece and, as aconsequence thereof, the amperage level. However, the amperage level canbe maintained at a set level, i.e. the amperage level is restored to itsprevious value following a change of the amperage level, throughautomatic adjustment of the hot wire feed speed. The CA welding processhas several advantages. For example, the maintenance of a given amperagelevel ensures that heat input and penetration remains essentiallyconstant throughout the welding process.

It has been discovered that a self-regulating CA welding process isunsuitable for use during the arc-stabilizing phase. Sudden currentvariations are common during the arc-stabilizing phase and correspondingadjustments of the hot wire feed speed, executed automatically inresponse to said variations, are slow in comparison and may have anegative impact on the arc stabilization process.

The solution to this problem is to feed the hot wire at constant feedspeed (CW) during at least one initial sub-phase and one main sub-phaseof the arc-stabilizing phase, and to feed the cold wire at constant feedspeed (CW) at least during one main sub-phase of the arc-stabilizingphase. That is, the self-regulating CA welding process is made inactiveduring the arc-stabilizing phase. This solution solves the abovementioned problem of the self-regulating CA process having a negativeimpact on the arc stabilization process. The CW welding process ensuresrapid generation of a stable arc. The self-regulation CA welding processis activated when the arc-stabilization process enters the stable arcphase. This solution is compatible with a CA welding apparatus, whereinthe self-regulating wire feed speed control is turned off during thearc-stabilizing phase.

As mentioned above, the presence of one or more cold wires in thevicinity of the hot wire makes it even more difficult to generate astable arc between the hot wire and the work piece.

One solution to this problem is to feed the cold wire at constant feed(CW) speed during at least one sub-phase of arc-stabilizing phase. Aconstant cold wire feed speed has a less negative impact on thearc-stabilization process.

Another solution, compatible with the above described solution, is toensure that the negative impact of the cold wire is minimized oreliminated during at least the beginning of the arc-stabilizing phase.This is achieved by ensuring that the cold wire does not reach the weldpuddle during at least one initial sub-phase of the arc-stabilizingphase, or at least does not reach the weld puddle at a feed speed atwhich the cold wire may have a (significant) negative impact on thearc-stabilizing process. Preferably, the cold wire feed speed in adirection towards the work piece remains equal to or below 9 cm/minduring said initial sub-phase. More preferably, the cold wire remainsstationary (not fed forward) during said initial sub-phase.

Thus, the method according to the invention ensures a quick generationof a stable arc, and consequently a good weld quality from the beginningof the welding process, also when the welding apparatus is arranged tofeed one or more cold wires towards the work piece.

Advantageously, the feed speed of a cold wire is determined independence on at least one corresponding hot wire feed speed, i.e. thecold wire feed speed of a sub-phase is determined in dependence on atleast one hot wire feed speed of said sub-phase, to ensure that the coldwire assumes a feed speed suitable for the current melting rate.Additional welding parameters may be used to determine suitable coldwire feed speeds.

The feed speed of the hot wire, as well as the feed speed of the coldwire may vary from one sub-phase to another, as long as they remainconstant throughout said sub-phases.

“Stable arc phase welding current” is a recurring term. The stable arcphase welding current is defined as the welding current to be maintainedduring the part of the stable arc phase coinciding with the weldingphase. Said part of the stable arc phase may constitute the entirestable arc phase.

Another recurring term is “expected stable arc phase feed speed of thehot wire”. The expected stable arc phase feed speed of the hot wire, orthe expected stable arc phase hot wire feed speed, is defined as the hotwire feed speed corresponding to the stable arc phase welding current,that is, the feed speed required to maintain the stable arc phasewelding current at its set level under a predetermined set of weldingconditions. Note that the actual hot wire feed speed may vary during thestable arc phase whereas the expected stable arc phase hot wire feedspeed is a predetermined value used only to determine wire feed speedssuitable for the arc-stabilizing phase.

In some embodiments, it is desirable to initiate the welding phase asearly as possible, and most preferably immediately following the arcignition phase, simultaneously with the arc-stabilizing phase. In thisembodiment, advantageously, the feed speed of the hot wire remainsconstant throughout the arc-stabilizing phase, and even moreadvantageously at a level in the range 80-95% of the expected stable arcphase feed speed of the hot wire. This solution ensures that the weldproduced during the arc-stabilizing phase is of high quality and thatthe transition from the arc-stabilizing phase to the stable arc phase isexecuted as quickly and smoothly as possible (a large increase orreduction of the hot wire feed speed at the end of the arc-stabilizationphase may cause the arc to become unstable).

In other embodiments, it may be advantageous to delay the start of thewelding phase by executing one or more, usually two, start-up phasesbetween the ignition phase and the welding phase. The start-up phasesare introduced to ensure that a stable arc is present at the beginningof the welding phase. Each sub-phase, with the possible exception of thelast sub-phase, begins and ends simultaneously with a correspondingstart-up phase.

The feed speed of the hot wire may vary between at least two differentsub-phases. For example, the hot wire feed speed may be increased everytime the arc-stabilization process enters a new sub-phase. This isadvantageous in that it is usually easier to establish a stable arc at alow hot wire feed speed, and in that it is desirable to apply a hot wirefeed speed during the last sub-phase close to the expected hot wire feedspeed of the stable arc phase, to ensure a smooth transition from thearc-stabilizing phase to the stable arc phase. However, there areembodiments wherein the expected hot wire feed speed of the stable arcphase is too low to be suitable for generation of a stable arc. In theseembodiments, advantageously, the hot wire feed speed is graduallydecreased during the arc-stabilizing phase, from a high initial feedspeed value during the initial sub-phase appropriate for stable arcgeneration to a lower feed speed during the last sub-phase.

Of course, the hot wire feed speed does not have to gradually increaseor decrease during the arc-stabilizing phase. The hot wire may also befed forward at the same constant feed speed during two or moresub-phases and at a different constant feed speed during an additionalsub-phase.

Advantageously, the cold wire feed speed is significantly lower than thecorresponding hot wire feed speed during the initial start-up phase, tofacilitate generation of a stable arc early in the arc-stabilizationprocess. More advantageously, the cold wire remains stationary duringthe initial start-up phase. Thereafter, the cold wire feed speed may bedetermined in dependence on at least the feed speed of at least one hotwire, i.e. the cold wire feed speed is increased as the hot wire feedspeed is increased and reduced when the hot wire feed speed is reduced.

Note that the arc-stabilizing phase may comprise any number of initialand main sub-phases.

Suitable feed speed values for the hot wire during the arc-stabilizingphase can be determined in a plurality of ways. In one embodiment,without start-up phases, the user enters a welding current to bemaintained during the stable arc phase. This welding current is referredto as the stable arc phase welding current. The control unit uses thisinformation, and possibly other given welding parameters (e.g. arcvoltage and wire travel speed), and a predetermined table of values todetermine a hot wire feed speed to be applied during the arc-stabilizingphase. This hot wire feed speed is, advantageously, in the range 80-95%of the expected stable arc phase hot wire feed speed corresponding tosaid stable arc phase welding current.

Other embodiments may comprise start-up phases, wherein each start-upphase corresponds to a sub-phase of the arc-stabilizing phase. Herein,the user may enter one or more welding parameter values related to aspecific start-up phase as wells as a stable arc phase welding currentlevel to be maintained during the stable arc phase, and possibly otherwelding parameter values, and the control unit uses this information anda predetermined table of values to determine suitable hot wire feedspeeds for said start-up phases. Each start-up phase corresponds to asub-phase, so the suitable hot wire feed speed for a start-up phase isalso the suitable hot wire feed speed for the corresponding sub-phase.Examples of suitable welding parameters are welding current, arc voltageand travel speed of the hot and cold wires (note that most CA weldingapparatuses do not allow the user to enter hot wire feed speeds).

The arc-stabilizing phase may in some embodiments end after the end ofthe last start-up phase. The suitable hot wire feed speed for the periodbetween the end of the last start-up phase and the beginning of thewelding arc-phase, also called the final sub-phase, is determined asdescribed above with reference to an embodiment without start-up phases.

The arc-stabilizing phase may also end before the end of the laststart-up phase. The suitable hot wire feed speed for the period betweenthe end of the arc-stabilizing phase and the end of the last start-upphase may be determined by the control unit using the stable arc phasewelding current, a predetermined table of values, welding parametersspecific for the last start-up phase and possibly other weldingparameters entered by the user.

The skilled person realizes that there are many alternative ways todetermine suitable hot and cold wire feed speeds for the arc-stabilizingphase and that the scope of protection provided by the claims coverssaid alternative ways.

Advantageously, the feed speed of a hot wire during a sub-phase is inthe range 0-200% of the expected stable arc phase feed speed of said hotwire, and the constant feed speed of a cold wire during said sub-phaseis in the range 0-100% of the feed speed of said hot wire during saidsub-phase.

As mentioned above, there are circumstances under which the hot wire mayassume a feed speed during the arc-stabilizing phase higher than theexpected stable arc phase feed speed of said hot wire. It is alsopossible for the cold wire to assume a feed speed higher than acorresponding hot wire feed speed during the arc-stabilizing phase.

However, it is usually advantageous for the hot wire to assume a feedspeed during a sub-phase in the range 0-100% of the expected stable arcphase hot wire feed speed. It has been discovered that long arcs aremore easily stabilized. One way to increase the length of the arcs is toreduce the hot wire feed speed. Consequently, the feed speed of the hotwire is advantageously set at a value lower than the expected stable arcphase feed speed of said hot wire. A low feed speed also has thebeneficial effect of reducing the risk of the hot wire being burned off,which can happen when an arc is ignited and a high current istransferred through the hot wire. The risk of this happening during thearc-stabilizing phase is reduced if the welding current is set to arelatively low value. Yet another reason for applying a low feed speedto the hot wire is to reduce the risk of the hot wire hitting the bottomof the weld puddle. An inconsistent welding start may lead to a reducedmelting rate and a reduction of the distance between the hot wire endand the work piece. In a worst case scenario, the hot wire may come intocontact with the work piece. The reduced feed speed of the hot wirereduces the risk of this happening.

It is advantageous if the hot wire during the last sub-phase assumes afeed speed in the range 80-95% of the expected stable arc phase feedspeed of the hot wire, to facilitate a smooth transition from thearc-stabilizing phase to the stable arc phase and possibly also toensure a good weld quality during the last sub-phase. A hot wire feedspeed in this range also ensures that the welding current will besufficiently high at the beginning of the stable arc phase to produce agood weld.

Advantageously, the constant feed speed of the cold wire during asub-phase is in the range 0-200%, and more advantageously 0-100%, of theconstant feed speed of a hot wire during said sub-phase. Advantageously,at least during the last sub-phase, the cold wire feed speed is in therange 70-90% of the constant feed speed of said hot wire. However, thecold wire may adapt a lower feed speed, for example in the range 10-60%of a corresponding hot wire feed speed, during earlier sub-phases. Ofcourse, the cold wire feed speed is advantageously below 9 cm/min andeven more preferable 0 cm/min at least during the first sub-phase.

The melting rate of the cold wire will remain low as long as there is nostable arc present in the vicinity of the cold wire. Therefore, it isadvantageous if the feed speed of the cold wire is set lower than whatwould have been appropriate in the presence of a stable arc, to ensurethat the cold wire does not hit the bottom of the weld puddle. However,as with the hot wire, it is advantageous if the adjustment of the coldwire feed speed when the arc-stabilization process enters the stable arcphase is relatively small, especially considering that the cold wirefeed speed usually is not increased as quickly as the hot wire feedspeed.

Advantageously, the arc-stabilizing phase has a predetermined length.This eliminates the need for stable arc detection means and the weldingapparatus becomes simpler and less expensive. Advantageously, thearc-stabilizing phase has a predetermined length in the range 1-6seconds and preferably in the range 1-3 seconds.

Advantageously, when the hot wire feed speed remains constant throughoutthe arc-stabilizing phase, the cold wire feed speed remains equal to orbelow 9 cm/min during a first portion of the arc-stabilizing phase,which first portion preferably has a predetermined length in the range0.5-5 seconds and more preferable in the range 2-3 seconds. That is,advantageously, the length of the initial sub-phase (or the combinedlength of a plurality of initial sub-phases) is in the range 0.5-5seconds and more preferably in the range 2-3 seconds.

When the hot wire feed speed does not remain constant throughout thearc-stabilizing phase, when it changes from one sub-phase to another,then the length of the initial sub-phase or the combined length of aplurality of initial sub-phases is, preferably, in the range 0.5-5seconds and more preferably in the range 1.5-2.5 seconds.

The invention is not limited to the above defined ranges. For example,the length of the arc-stabilizing phase may be as short as 0.1 seconds,in which case the length of the initial sub-phase(s) is adjustedaccordingly. The length of the arc-stabilizing phase may also be longerthan 6 seconds. The length of the initial sub-phase(s) may be bothshorter than 0.5 seconds and longer than 5 seconds.

Advantageously, when one or more start-up phases are included in thewelding process, the length of the initial sub-phase (or the combinedlength of a plurality of initial sub-phases) is shorter than the lengthof the main sub-phase (or the combined length of a plurality of mainsub-phases).

It is possible to provide the welding apparatus with some sort of arcdetection means, which detects the presence of a stable arc and sendsinformation indicating the presence of a stable arc to a control unitthat executes the switch from CW to CA. This solution ensures that thearc-stabilizing phase remains as short as possible and at the same timeensures that the switch to CA is not executed too early, before a stablearc has been produced.

The method advantageously comprises the step of feeding at least oneadditional hot wire towards the work piece while transferring weldingcurrent to said additional hot wire for arc generation. Advantageously,the feed speed of an additional hot wire is regulated as describedabove.

Advantageously, the method comprises the step of igniting an arc betweenthe at least one additional hot wire and the work piece. Advantageously,the initial and main sub-phases all comprise the step of feeding said atleast one additional hot wire towards the work piece at constant feedspeed, wherein the constant feed speed of one sub-phase may be the sameor different from the constant feed speed of another sub-phase.Advantageously, the stable arc phase comprises the step of continuouslyadjusting the feed speed of said at least one additional hot wire independence on at least a welding current transferred through said hotwire.

Incorporating one or more additional hot wires into the welding processis a means to increase the deposition rate. With this arrangement, italso becomes possible to assign different task to the hot wires.However, arranging a plurality of hot wires in the vicinity of a coldwire may also result in a plurality of sparks striking at the same timeduring the arc-stabilizing phase. This problem is particularly common intwin setups where two wires are connected to one power source. Thepresence of a plurality of sparks makes it difficult for the arcs tostabilize and if no stable arcs are in place to melt the cold wire, thecold wire may hit the plate and buckle, causing the welding equipment tosway. Thus, it becomes even more important to ensure the generation of astable arc when the welding process includes one or more hot wires. Thisis achieved with the method and welding apparatus according to theinvention.

Advantageously, the feed speed of the cold wire during thearc-stabilizing phase is dependent on the feed speed of a single hotwire. This allows for a simple and less expensive solution. However, itis also possible to determine the feed speed of the cold wire independence on the feed speed of more than one hot wire, e.g. a meanvalue of a plurality of hot wire feed speeds.

During the stable arc phase, the feed speed of the cold wire can bedetermined in dependence on the feed speed(s) of one or more hot wires.For example, in an arrangement where two or more hot wires are locatedat a distance from one another along an axis extending in the weldingdirection (the direction of movement of the welding apparatus) and thehot wires are assigned different tasks, wherein the leading hot wire(located first as seen in the direction of welding) is used to controlthe degree of penetration and the trailing hot wires (located behind theleading hot wire as seen in the direction of welding) are used tocontrol bead appearance, contour and fill, the feed speed of the coldwire is advantageously related to the feed speed of one or more of thetrailing hot wires (e.g. a mean value of a plurality of trailing hotwire feed speeds). In an alternative embodiment, the feed speed of thecold wire may be related to the feed speed of the leading hot wire or amean value of the feed speeds of leading and trailing hot wires.Additional ways of arranging and relating the feed speed of one or morecold wires to one or more hot wires are possible. For example, in twinwelding, wherein two or more hot wires are connected to the same powersource, the hot wires are usually considered as a single hot wire andthe feed speed of the cold wire may be related to the feed speed of asingle hot wire.

Other suitable methods for continuous adjustment of hot and cold wirefeed speeds during the stable arc phase are described inPCT/EP2012/003461, the content of which is hereby incorporated byreference.

The feed speed of the cold wire can also be related to additionalwelding parameters, e.g. arc voltage and welding head travelling speed.

Note that the feed speed of the cold wire can be indirectly related toone or more hot wire feed speeds. This is the case when the feed speedof the cold wire is related to an active welding parameter which in turnis related, directly or indirectly, to the feed speed of said hotwire(s).

In some embodiments, the at least one main sub-phase comprises the stepof feeding at least one additional cold wire towards the work piece at aconstant feed speed and the stable arc phase comprises the step ofcontinuously adjusting the feed speed of said at least one additionalcold wire in dependence on at least one hot wire feed speed. Oneadvantage with using more than one cold wire is an increased depositionrate.

The feed speeds of these additional cold wires can be regulated asdescribed above. It is also possible for one cold wire to adopt the feedspeed of another cold wire. The feed speeds of more than one cold wirecan be determined in dependence on the same welding parameters ordifferent welding parameters.

Advantageously, an increase or reduction of the cold wire feed speedduring the arc-stabilizing phase is carried out as quickly as possibleand preferably more or less instantaneously, to minimize the negativeeffect of the cold wire on the arc-stabilization process and, in someembodiments, to make sure that welding may be carried out as early aspossible with the best possible result.

Instantaneous adjustments are carried out as quickly as allowed by thewelding apparatus.

Hot and cold wire feed speed adjustments during the arc-stabilizingphase are executed as quickly as possible. Preferably, a wire reachesits new feed speed within 100 ms and more preferably within 10 ms. Thetime required for the wire to reach its target feed speed depends onseveral parameters, e.g. the diameter of the wire and the type of motorarranged to drive the wire feeding means. Nevertheless, a wire may for ashort period of time at the beginning and/or end of a sub-phase have afeed speed that differs from the otherwise constant feed speed of saidsub-phase. In this specification, a wire is considered to be fed at aconstant feed speed throughout a sub-phase even if the feed speed ofsaid wire is adjusted at the very beginning and/or end of saidsub-phase.

An arc may become unstable following an adjustment of the hot wire feedspeed. This may lead to a temporarily reduced melting rate. During thestable arc phase, it is advantageous if the cold wire feed speed isincreased at a rate lower than the corresponding increase rate of thehot wire feed speed. This will ensure that the cold wire does not hitthe bottom of the weld pool due to a temporarily reduced melting rate.One way of ensuring that the cold wire feed speed is not increased tooquickly during the stable arc phase is to increase the cold wire feedspeed in steps until the cold wire has reached its target value. Thecold wire feed speed is advantageously increased in steps having a meanheight of up to 100 cm/min, advantageously 1-10 cm/min and even moreadvantageously 4-6 cm/min, and a mean length of 10-1000 ms,advantageously 50-500 ms and most preferably 75-125 ms. It is alsopossible to delay the initiation of the increase of the cold wire feedspeed. A reduction of the cold wire feed speed during the stable arcphase should be executed as quickly as possible, to ensure that the coldwire feed speed is immediately adapted to a lower melting rate.Advantageously, the new and lower target value for the cold wire feedspeed is reached within 200 ms, preferably within 100 ms, morepreferably within 10 ms and most preferably within 1 ms with respect tothe occurrence of the hot wire feed speed adjustment causing saidreduction.

The welding apparatus may be provided with one or more means formeasuring the welding current, or an active welding parameter related tothe welding current, during the stable arc phase. These values arefiltered and used to determine new target values for the hot wire feedspeed.

The welding apparatus may also be provided with one or more measuringmeans arranged to measure a hot wire feed speed, or one or more activewelding parameters related to and indicative of the hot wire feed speed,during the welding process. Measured values are stored in the controlunit. The time interval between active welding parameter measurements isadvantageously as short as possible, at least during the stable arcphase when the hot wire feed speed is continuously adjusted independence on welding current variations. A suitable time interval isabout 1 ms. The measured values may be filtered to achieve a moreaccurate adjustment of the cold wire feed speed. A filtered value iscompared to the last stored value and thus it can be determined whetherthe cold wire feed speed should be increased or reduced. The lastmeasured filtered value is used to determine a new target value for thecold wire feed speed. Active welding parameter values used to determinecorresponding target values for the cold wire feed speed areadvantageously measured at intervals having a mean length of 10-1000 ms,preferably 50-500 ms and most preferably 75-125 ms. Consequently, thecold wire feed speed may be adjusted in steps having a mean length of10-1000 ms, advantageously 50-500 ms and most preferably 75-125 ms. Itis of course possible to increase the time interval between themeasurements of the active welding parameter to up to 1000 ms. Should anew welding parameter value, measured during a hot wire feed speedadjustment, indicate that the welding parameter has changed during saidadjustment, then a new target value for the hot wire feed speed isdetermined and the hot wire feed speed is adjusted accordingly.Similarly, a detected change in hot wire feed speed during a cold wirefeed speed adjustment will lead to an adjustment of the cold wire feedspeed target value.

It is possible, during the stable arc phase, to reduce the cold wirefeed speed to a level below the target value, to ensure that the arc isgiven sufficient time to stabilize and that the cold wire does not hitthe bottom of the weld puddle, and then increase the cold wire feedspeed to the target value. The cold wire feed speed may even be broughtto a halt before being increased to said target value.

Advantageously, the control unit adapted to determine new target valuesfor the cold wire feed speed immediately following a transition from thearc-stabilizing phase (CW) to the stable arc phase (CA) is adapted tocompare the hot wire feed speed of the last sub-phase of thearc-stabilizing phase to the hot wire feed speed first measured duringthe stable arc phase. An increase of the cold wire feed speed at thetransition from the arc-stabilizing phase to the stable arc phase can bedelayed, for example with 0.5-1.5 seconds, to ensure that a stable arcis present when the cold wire reaches its new, higher target value. Areduction of the cold wire feed speed following the transition from thearc-stabilizing phase to the stable arc phase is, preferably,instantaneous, to prevent the cold wire from hitting the weld puddlebefore it reaches its new, lower target value.

The welding apparatus may also be provided with other measuring means,for measuring other welding parameters.

The second object of the invention is achieved by means of a weldingapparatus for carrying out the above described method. The weldingapparatus comprises a hot wire feeding means for feeding at least onehot wire towards a work piece, a contact means for transferring weldingcurrent to said hot wire for arc generation, a cold wire feeding meansfor feeding at least one cold wire towards said work piece and a controlunit adapted to control said hot and cold wire feeding means during anarc ignition phase, an arc-stabilizing phase and a subsequent stable arcphase. The arc-stabilizing phase comprises at least one initialsub-phase and at least one subsequent main sub-phase. The control unitis adapted to control the hot wire feeding means to feed the hot wire ata constant feed speed during the initial sub-phase, feed the hot wire ata constant feed speed during the main sub-phase and to continuouslyduring the stable arc phase adjust the feed speed of the hot wire independence on at least a welding current transferred through said hotwire. The control unit is also adapted to control said cold wire feedingmeans to feed at least one cold wire at a constant feed speed during themain sub-phase and to continuously during the stable arc phase adjustthe feed speed of the cold wire in dependence on at least one hot wirefeed speed.

The welding apparatus according to the invention ensures high weldquality also at the beginning of a welding process including one or morecold wires. This is achieved by feeding the hot wire at constant wirefeed speed during at least one initial sub-phase and one main sub-phaseof the arc-stabilizing phase and by feeding the cold wire at a constantwire feed speed during at least one main sub-phase of thearc-stabilizing phase, to ensure a relatively quick generation of astable arc between the hot wire and the work piece (or a plurality ofarcs between a plurality of hot wires and said work piece).

The welding apparatus also makes it possible to minimize or eliminatethe negative impact of the cold wire during at least the beginning ofthe arc-stabilizing phase. The control unit prevents the cold wire fromreaching the weld puddle during at least one initial sub-phase of thearc-stabilizing phase, or at least prevents it from reaching the weldpuddle at a feed speed at which the cold wire may have a seriousnegative impact on the arc-stabilizing process. Preferably, the controlunit is adapted to maintain the cold wire feed speed in a directiontowards the work piece equal to or below 9 cm/min during at least theinitial sub-phase. More preferably, the control unit is adapted to keepthe at least one cold wire stationary during the initial sub-phase. Thisarrangement ensures that the arc-stabilization process is not negativelyinfluenced by the cold wire during the first stage(s) of thearc-stabilizing phase.

The control unit may be adapted to control said hot wire feeding meansto provide one hot wire feed speed during one first sub-phase andanother hot wire feed speed during another sub-phase. This arrangementmakes it possible to stepwise increase or reduce the hot wire feed speed(and thus also the cold wire feed speed) during the arc-stabilizingphase, which may have a positive effect on the arc-stabilizationprocess.

Alternatively, the control unit may be adapted to provide the same hotwire feed speed throughout the arc-stabilizing phase, which makes itpossible to initiate the welding phase at the same time as thearc-stabilizing phase. In this case, it is advantageous if the feedingof the cold wire is delayed, preferably with 0.5-5 seconds and morepreferably with 2-3 seconds, to facilitate a quick generation of astable arc.

The control unit may be adapted to determine an expected stable arcphase feed speed of the hot wire corresponding to a stable arc phasewelding current. Advantageously, the hot wire during the arc-stabilizingphase is in the range 0-200%, preferably in the range 0-100%, of saidexpected stable arc phase hot wire feed speed. It is even morepreferable if the control unit is adapted to keep the feed speed of thehot wire during the last sub-phase of the arc-stabilizing phase in therange 80-95% of the expected stable arc phase hot wire feed speed.

A hot wire feed speed lower than the expected stable arc phase hot wirefeed speed ensures that the distance between the tip of the hot wire andthe work piece remains large, which facilitates the generation of astable arc. For this reason, it is advantageous if the hot wire feedspeed(s) remains below the expected stable arc phase hot wire feed speedduring the entire arc-stabilizing phase. For this reason, it may also beadvantageous if the hot wire feed speed is significantly lower than theexpected stable arc phase hot wire feed speed during the first part ofthe arc-stabilizing phase. However, it is also advantageous if thedifference between the expected stable arc phase hot wire feed speed andthe constant hot wire feed speed during the last sub-phase is relativelysmall, to facilitate the transition from the arc-stabilizing phase tothe stable arc phase. Thus, it is advantageous if the hot wire feedspeed can be adjusted in steps during the arc-stabilizing phase. Once astable arc has been generated, it is no longer necessary to maintain alow hot wire feed speed and the welding apparatus switches to a CAprocess wherein the hot wire feed speed is adjusted to maintain thewelding amperage at a set level (the stable arc welding current).

It may also be advantageous if the hot wire feed speed is higher thanthe expected stable arc phase hot wire feed speed, and perhaps alsogradually reduced during the arc-stabilizing phase, to ensure thegeneration of a stable arc. This is the case when the stable arc phasewelding current is too low to ensure that a stable arc will begenerated.

Advantageously, each constant feed speed of the cold wire during thearc-stabilizing phase is in the range 0-200%, preferably in the range0-100%, of a corresponding hot wire feed speed. The control unit mayalso be adapted to keep the cold wire feed speed, at least during thelast sub-phase, in the range 70-90% of the corresponding hot wire feedspeed, to facilitate the transition from the arc-stabilizing phase tothe stable arc phase.

Advantageously, a user enters one or more welding parameter valuesbefore the welding process begins. Examples of suitable weldingparameters are welding current, arc voltage and hot wire travellingspeed. The control unit uses said welding parameter values to determinesuitable feed speeds for the hot and cold wires.

In embodiments wherein the hot wire feed speed may change from sub-phaseto sub-phase, the user may enter sub-phase-specific welding parametervalues to be used to determine a suitable hot wire feed speed for eachsub-phase. It is also possible, in alternative embodiments, for the userto enter suitable hot and cold wire feed speeds for the arc-stabilizingphase.

Advantageously, the control unit is adapted to provide thearc-stabilizing phase with a predetermined length in the range 1-6seconds and more advantageously in the range 1-3 seconds, to ensure thata stable arc is present when the welding process enters the stable arcphase.

The initial sub-phase advantageously has a predetermined length in therange 0.5-5 seconds.

The welding apparatus may comprise at least one stable arc detectionmeans arranged to detect a stable arc and the control unit may beadapted to initiate the stable arc phase when it receives a signal fromsaid stable arc detection means indicating that a stable arc has beendetected. This allows for an optimization of the length of thearc-stabilizing phase.

The welding apparatus may comprise one or more measuring means arrangedto measure one or more active welding parameter values. These measuringmeans may form part of the control unit but they may also be separatelyarranged, in which case they are connected to the control unit so as tobe able to send relevant information to said control unit.

Said measuring means may be arranged to measure the hot wire feed speed.For example, said measuring means may comprise a sensor adapted tomeasure the rotational speed of a motor shaft in a motor arranged tofeed a hot wire towards a work piece and transfer this information tosaid control unit, which calculates the hot wire feed speed. It is alsopossible to use one or more sensors that measure the feed speed directlyon the hot wire.

One or more measuring means may be adapted to measure the weldingcurrent. For example, said measuring means may comprise one or moreshunts in the power source. The shunt is placed in series with a load sothat all of the current to be measured will flow through it. The voltagedrop across the shunt is proportional to the current flowing through itand the shunts resistance is known, wherefore measuring the voltageallows for determination of the welding current.

Said measuring means may also be adapted to measure arc voltage. Arcvoltage is advantageously measured between the work piece and theclosest end of the hot wire to avoid voltage drop.

Of course, said measuring means can be arranged to measure other activeor non-active welding parameters and the control unit may use thesemeasured welding parameter values to determine, for example, weldingcurrent, arc voltage and hot wire travelling speed, which may be used todetermine suitable hot and cold wire feed speeds.

The welding apparatus may also comprise at least one arc detection meansadapted to detect the presence of an arc during the arc ignition phase.In this embodiment, the control unit is advantageously adapted toinitiate the arc-stabilizing phase immediately upon receiving a signalfrom said arc detection means indicating the presence of an arc.

For example, an arc is considered to be established when the weldingcurrent amperage level exceeds a pre-set level for a time intervalexceeding a pre-set time. This detection means may form part of thecontrol unit. It may also be separate from and connected to the controlunit.

The welding apparatus may comprise one or more hot wire feeding meansfor feeding one or more hot wires towards the work piece and additionalcontact means for transferring welding current to said hot wires. Thecontrol unit is suitably adapted to control said hot wire feeding meansto feed said hot wires at constant feed speeds during the at least oneinitial sub-phase, at constant feed speeds during the at least one mainsub-phase and to continuously during the stable arc phase adjust thefeed speeds of each of said hot wires in dependence on at least awelding current transferred through said hot wire.

The welding apparatus may also comprise one or more cold wire feedingmeans for feeding one or more cold wires towards the work piece. Thecontrol unit is suitably adapted to control said cold wire feeding meansto feed said cold wires at constant feed speeds during the at least onemain sub-phase, and to continuously adjust the feed speeds of said coldwires in dependence on at least one corresponding hot wire feed speedduring the stable arc phase.

Advantageously, any additional hot and cold wire feeding means iscontrolled in the same manner as described with reference to the otherhot and cold wire feeding means.

The addition of one or more hot wires increases the deposition rate.However, the addition of additional hot wires may also make thegeneration of stable arcs during the arc-stabilizing phase moredifficult. An arc generated by a hot wire may affect arcs generated byother hot wires and prevent these arcs from stabilizing. The weldingapparatus according to the invention solves this problem by feeding thehot wires at constant feed speeds during sub-phases of thearc-stabilizing phase, thus creating welding conditions that facilitatesgeneration of stable arcs. The constant hot wire feed speeds are chosenso that stable arcs may be easily generated.

A hot wire feeding means may be adapted to feed one or more hot wirestowards the work piece. That is, a hot wire feeding means may in oneembodiment be adapted to feed a single, first hot wire towards a workpiece and in an alternative embodiment be adapted to feed additional hotwires in addition to said first hot wire. It is also possible to usemore than one hot wire feeding means, each adapted to feed one or morehot wires, in the same welding apparatus.

Note that two or more hot wires in a welding apparatus need not have thesame feed speed, neither during the sub-phases of the arc-stabilizingphase nor during the stable arc phase, and need not have the same wirecharacteristics, e.g. material properties and wire dimensions. Suitablehot wire feed speeds may be determined individually for each hot wire.It is also possible to determine suitable hot wire feed speeds for onehot wire and apply said feed speeds to one or more additional hot wires.

The presence of additional hot wires provides alternatives for how toregulate the feed speed of a single cold wire. The feed speed of asingle cold wire may, during the arc-stabilizing phase, be dependent onthe feed speed of a single hot wire or an average feed speed of aplurality of hot wires. The hot wire feed speed values may be weightedbefore the average hot wire feed speed is calculated. Similarly, duringthe stable arc phase, the feed speed of a single cold wire may bedetermined in dependence on one or more active welding parametersindicative of the feed speeds of one or more hot wires. The cold wirefeed speed may, for example, be related to a single hot wire feed speedor to a plurality of hot wire feed speeds.

The control unit may be a single unit or comprise a plurality ofsub-units located at different locations.

As mentioned, the welding apparatus may also comprise cold wire feedingmeans for feeding a plurality of cold wires towards the work piece. Twoor more cold wires in a welding apparatus need not have the same feedspeed, neither during the sub-phases of the arc-stabilizing phase northe stable arc phase. For example, the individual feed speeds of twocold wires may be related to different welding parameters, e.g. the feedspeeds of different hot wires. It is also possible to determine the coldwire feed speed for a single cold wire and apply the same feed speed toall cold wires. It is also possible to use the same welding parametervalues when the feed speeds of two or more cold wires are calculated,which cold wires may have different dimensions and/or materialproperties and serve different functions and thus be assigned differentfeed speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above-mentioned and otherobjects and advantages may best be understood from the followingdetailed description of exemplary embodiments of the invention. Thedetailed description contains references to drawings, wherein:

FIG. 1 shows a twin wire welding apparatus according to the invention;

FIG. 2 shows a welding head according to the invention;

FIG. 3 shows the welding head in FIG. 2 turned counter-clockwise by 90°;

FIG. 4 shows a perspective view or the arc-welding head in FIG. 2 ;

FIG. 5 shows a perspective view of an arc-welding welding head accordingto the invention;

FIG. 6 a, b shows schematically different phases of twoarc-stabilization process; and

FIG. 7 a, b shows how hot and cold wire feed speeds may vary over time.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the drawings, equal or similar elements are referred to by the samereference numerals. The drawings are merely schematic representationsand not intended to portray specific parameters of the invention.Moreover, the drawings are intended to depict only typical embodimentsof the invention and therefore should not be considered as limiting thescope of the invention.

FIG. 1 shows portions of a twin welding apparatus 1. The twin weldingapparatus includes a first contact tube 2 for guiding a first hot wire 4towards a weld puddle 6. The first contact tube 2 is arranged in acontact tip 8 in a conventional manner. Welding current is transferredthrough said contact tube 2 to the first hot wire 4. A second contacttube 10 is arranged in the twin wire welding apparatus 1 for guiding asecond hot wire 12 towards the weld puddle 6. The second contact tube 10is arranged in a contact tip 14 in a conventional manner. The first andsecond contact tips 8, 14 may be arranged in a single body, which may beaggregated by parts, or in separate bodies. At the second contact tube10, welding current is transferred to the second hot wire 12.

A single power source 16 is connected to a contact device 18 includingthe contact tips 8, 14 and housing the first and second contact tubes 2,10. The single power source 16 provides the same potential to the firstand second hot wires 4, 12. The power source may be of any conventionaltype operable for twin wire welding, such as a welding converter, awelding transformer, a rectifier, a thyristor controlled rectifier or aninverter.

The twin wire welding apparatus 1 further includes a feeding arrangementfor feeding a cold wire 22 into the weld puddle 6. The feedingarrangement includes a tube 24, which is electrically insulated from thefirst and second contact tips 8, 14. The cold wire 22 is fed via thetube 24. When welding, arcs 40 will be present at the first and secondhot wires 4, 12, but not at the cold wire 22. The cold wire 22 is meltedby introduction of the cold wire into areas of the arcs 40. Suitably,the cold wire 22 is not connected to any electrical power source andwill therefore generally assume a ground potential. However, it may bepossible to connect the cold wire 22 to a power source for pre-heatingthe cold wire. However, the cold wire 22 will not be connected to apower source for the purpose of arc generation. The tube 24 may be ametallic tube that is isolated from the first and second contact tips 8,14, or a ceramic tube.

In submerged arc welding an arc is present between the tip of a hot wireand the work piece. The arc and the melted material are protectedbeneath a layer of pulverized flux. The flux melts in part during theprocess, thus creating a protecting layer of slag on the weld puddle.

An arc 40 is shown in FIG. 1 . The contact of the arc 40 at the workingpiece will be moving in a random manner. However, normally it is assumedthat the arc 40 is present within a cone 42 extending from a tip 34 ofthe hot wire to the weld puddle 6. The opening angle β of the cone 42may vary from welding case to welding case. However, a normal openingangle β is around 30°. For this reason it is preferable to locate thecold wire 22 such that it enters the arc area in an essentiallyorthogonal direction thereto at an axial distance D being less thanL*cotan (β/2) from the consumable electrode measured at the tip 34 ofthe consumable electrode. Here L is the arc length, which is thedistance from the electrode tip 34 to the closes point 36 of the weldpuddle.

A flux hopper (not shown) is arranged to feed granular flux to a contactdevice 160 that holds the hot wires 4, 12 and the cold wire 22. Thegranular flux is fed to the contact device 160 via a nozzle (not shown).

It may be preferable to arrange the cold wire 22 in between the two hotwires 4, 12. The hot wires 4, 12 are preferably mounted at an axialdistance A being less than a cone diameter measured at the surface 30 ofthe weld puddle 6. With this arrangement, the cold wire 22 will beintroduced into the outer parts of the arc area defined by the cones 42of both hot wires 4, 12, which is beneficial for the weld result.

The twin wire welding apparatus 1 further comprises a sensor 27 formeasuring the feed speed of the first hot wire 4. Of course, it ispossible to employ more than one sensor and to measure the feed speed ofboth hot wires. However, in twin welding, when two hot wires areconnected to the same power source, the hot wires are often consideredas a single hot wire and only one sensor is required. In alternativeembodiments, the sensor 27 may be replaced by any suitable measuringmeans adapted to measure other welding parameters.

The twin wire welding apparatus 1 also comprises arc detection means(not shown) for detecting an (unstable) arc between the hot wire 4 andthe work piece.

The twin wire welding apparatus 1 in FIG. 1 is adapted to carry out awelding process comprising an ignition phase and a welding phase. Thetwin wire welding apparatus is a CA welding apparatus. The twin wirewelding apparatus 1 is also adapted to carry out an arc-stabilizationprocess, starting with an ignition phase, followed by an arc-stabilizingphase comprising two sub-phases (an initial sub-phase and a mainsub-phase), and a stable arc phase. The welding apparatus 1 is a CAwelding apparatus. However, the automatic self-regulation of the feedspeeds of the hot wires 4, 12 will be turned off during thearc-stabilizing phase so that the feed speeds of the hot wires 4, 12remain constant during each sub-phase of the arc-stabilizing phase. Notethat hot wires in a twin wire welding apparatus are considered as asingle hot wire; that is, the hot wires 4, 12 will always assume thesame feed speeds. The cold wire 22 will be fed towards the work pieceduring the main sub-phase only and the feed speed of the cold wire 22 isdependent on the measured feed speed of the first hot wire 4 during saidmain sub-phase. Consequently, the feed speed of the cold wire 22 willremain constant during the main sub-phase. The welding process carriedout during the stable arc phase is a CA welding process, during whichthe feed speed of the hot wires 4, 12 are adjusted to compensate fordisturbances to maintain the welding current amperage at an essentiallyconstant level.

Before the welding process begins, a user of the welding apparatus 1enters a welding current value to be maintained during the stable arcphase. This welding current value is called the stable arc phase weldingcurrent. The user may enter additional welding parameters, e.g. arcvoltage and wire travel speed. A control unit 31 uses said informationand a table of values to calculate an appropriate arc-stabilizing phasefeed speed value for the two hot wires 4, 12 (the hot wire feed speedremains constant throughout the arc-stabilizing phase) and a mainsub-phase feed speed value for the cold wire 22. Thereafter, the arcignition phase is initiated and the arc ignition phase is followed bythe initial sub-phase, initiated upon detection of the presence of anarc between the first hot wire and the work piece by means of said arcdetection means. In this embodiment, the arc-stabilizing phase has apredetermined length of 2.5 seconds. When the arc-stabilizing phase isover, it is assumed that a stable arc has been generated and that the CAwelding process may commence.

Observe that welding is carried out during both the arc-stabilizingphase and the stable arc phase.

During the stable arc phase, the sensor 27 continuously, with intervalsof about 1 millisecond, measures the feed speed of the hot wire 4 andtransfers measured hot wire feed speed values to the control unit 31.The control unit 31 filters the received values; the values to besubsequently used to control the feed speed of the cold wire 22 aremeasured at intervals having a mean length of between 75-125milliseconds. For each filtered value, the control unit 31 determines acorresponding feed speed target value for the cold wire 22.

The control unit 31 also determines whether the target value is higheror lower than the current cold wire 22 feed speed. A reduction of thecold wire 22 feed speed is advantageously carried out as quickly aspossible; whereas an increase of the cold wire feed speed should bedelayed with a time period dependent on the size of said increase, toensure that the arcs 40 are stable before the cold wire 22 feed speedreaches its target value.

A signal is sent from the control unit 31 to a cold wire feeding means35 disposed for feeding the cold wire 22 towards the work piece. Thecold wire feeding means 35 increases or reduces the feed speed of thecold wire 22 in accordance with instructions from the control unit 31.

FIGS. 2 to 4 depict different views of an electric arc-welding weldinghead 100 for the twin welding apparatus 1 in FIG. 1 .

At one end, the welding head 100 comprises a contact device 160, whichduring welding is in close proximity to the work piece to be welded. Thecontact device 160 holds a wire assembly 170 comprising the wires 4, 22,12 (only the cold wire 22 is shown in FIG. 2 ). The wires 4, 22, 12 exitthe contact device 160 through an outlet 162 at the lower end of thecontact device 160 facing the work piece during welding. The wires 4,22, 12 may be fed from respective reservoirs such as coils (not shown)towards the arc welding head 100.

As mentioned above, the wire assembly 170 comprises two hot wires 4, 12and a cold wire 22 arranged in the contact device 160. The hot wires 4,12 are arranged as so called twin wires, which are fed in parallel as adouble wire arrangement.

Above the contact device 160 a feeder means 150 is arranged which feedsthe hot wires 4, 12 towards the contact device 160. Typically, thefeeder means 150 comprises grooved wheels which move the hot wires 4, 12towards the contact device 160. The feeder means 150 comprises anelectrically insulating portion 156 for feeding through the cold wire22. The electrically insulating portion 156 can consist of feeder wheelswith an extra insulated groove for the cold wire 22. The cold wire 22can pass through the wire feeding means 150 freely. The feeder wheelsare driven by a driving unit 152 (not shown in FIG. 2 ), e.g. anelectric motor.

The flux hopper 11 feeds granular flux to the contact device 160 via anozzle (not shown).

Besides the driving unit 152 the wire feeding means 150 comprises a gearwith a drive shaft. On the drive shaft of the gear a feeding wheel 154(FIG. 5 ) is arranged, which can be pressurized by another wheel (notshown). The feeding wheel 154 drives the wire forward in the directionof the contact device 160.

A wire straightening unit 140 is arranged above the wire feeding means150 for straightening the hot wires 4, 12. Two rollers depicted in aforemost position of the wire straightening unit 140 are used to exert apressure on three fixed wheels arranged vertically one over the other inthe rear part of the wire straightening device. The pressure the rollersare exerting on the wheels is adjustable via knobs at the outside of thewire straightening unit 140. The pressure of the rollers on the threewheels is straightening the wire. The wire straightening unit 140comprises an electrically insulating portion 146 through which the coldwire 22 can pass freely through the wire straightening unit 140.

Above the wire straightening unit 140 a separate wire feeding means 35is disposed for feeding the cold wire 22 towards the contact device 160.On the wire feeding means 35 a driving unit 132, e.g. an electric motor,is arranged to drive feeder wheels of the wire feeding means 35. Besidesthe driving unit 132, the wire feeding means 35 comprises a gear with adrive shaft. On the drive shaft of the gear a feeding wheel 134 (FIG. 5) is arranged, which can be pressurized by another wheel (not shown).The feeding wheel 134 drives the cold wire 22 forward in the directionof the contact device 160.

Above the wire feeding means 35 a separate wire straightening unit 120is arranged for straightening the cold wire 22. Along the longitudinalextension of the welding head 100 an electrically insulating duct 180 isprovided for guiding the cold wire 22 from a wire reservoir such as awire bobbin (not shown) to the contact nozzle. Between the feeder means150 and 130 and above the wire straightening unit 120 an electricallyinsulated wire conduit can be arranged which receives the cold wire 22.

Particularly, the electrically insulating duct 180 consists of theelectrically insulating portion 146 of the wire straightening unit 140,the electrically insulating portion 156 of the wire feeding means 150for the non-insulated hot wires 4, 12, and the electrically insulatedportion of the contact device 160 as well as electrically insulated wireconduits between the units 130, 140, 150, 160 and above the wirestraightening unit 120 for the electrically insulated cold wire 22.

A detailed description of suitable contact devices for the hot and coldwires is provided in, for example, WO 2012/041375 A1.

As mentioned above, the arc welding apparatus 1 is provided with asensor for measuring the feed speed of the hot wire 4.

The arc welding apparatus 1 is also provided with arc detection means(not shown) for detecting the presence of an arc between the first hotwire and the work piece.

FIG. 5 is a side view of an arc-welding welding head 100 of virtuallythe same layout as shown in FIGS. 2-4 . Above the wire straighteningunit 140 two guide tubes 142, 144 are provided for twin wires. The guidetubes 142, 144 are arranged crosswise to the longitudinal extension ofthe welding head 100. A guide tube 182 for the cold wire is arrangedbetween the wire feeding means 35 for the cold wire (not shown) and thewire straightening unit 140 for the hot wires (not shown). The drivingunits 132, 152 can be equipped with pulse sensors for speed control ofthe wires. Close to the contact device 160 a nozzle 116 for a fluxhopper 11 (FIGS. 2-4 ) is arranged. The nozzle 116 is fixed to a rod 118arranged parallel to the longitudinal axis of the contact device 160.

FIG. 6 a shows schematically the different phases of the submerged arcwelding process to be carried out by means of the arc welding apparatussimilar to the arc welding apparatus shown in FIG. 1 . The cold wirefeed speed is determined in dependence on the feed speed of the firsthot wire (the same as the feed speed of the second hot wire).

The submerged arc welding process comprises an ignition phase, duringwhich a first (unstable) arc is ignited between the first hot wire andthe work piece. The ignition phase is immediately followed by a weldingphase, during which welding is carried out on the work piece. Thewelding phase begins as soon as an arc has been detected.

The welding apparatus is also adapted to carry out an arc-stabilizationprocess comprising an ignition phase (IP), an arc-stabilizing phase (AP)and a stable arc phase (SP). The arc-stabilizing phase (AP) consists ofan initial sub-phase (IS) and a main sub-phase (MS). The arc-stabilizingphase (AP) and the welding phase begin simultaneously, that is, weldingis carried out during the arc-stabilizing phase (AP). For this reason,it is essential that a stable arc is generated as soon as possible. Inthis embodiment, the arc-stabilizing phase (AP) has a predeterminedlength of 2.5 seconds (deemed sufficient to generate a stable arc). Theinitial sub-phase (IS) has a predetermined length of 2.0 seconds and themain sub-phase (MS) has a predetermined length of 0.5 seconds.

Before the ignition phase (IP) is initiated, the user of the weldingapparatus enters a set of welding parameter values including a stablearc phase welding current to be maintained during the stable arc phase(SP). The control unit then determines a hot wire feed speed suitablefor the arc-stabilizing phase (AP) to be applied to the first and secondhot wires. Note that the hot wire feed speed remains constant throughoutthe arc-stabilizing phase (AP). Once the arc detection means hasdetected an arc between the first hot wire and the work piece, thecontrol unit initiates the arc-stabilizing phase (AP). During theinitial sub-phase (IS), the hot wires are fed forward at the feed speeddetermined by means of the control unit whereas the cold wire remainsstationary (not fed forward). During the subsequent main sub-phase (MS),the hot wires are fed forward at said constant feed speed and the coldwire is fed forward at a constant feed speed determined in dependence onthe constant feed speed of the first hot wire. After 2.5 seconds haveelapsed since the beginning of the arc-stabilizing phase (AP), thearc-stabilization process enters the stable arc phase (SP).

The stable arc phase (SP) follows immediately after the arc-stabilizingphase (AP). The feed speed of the hot wires is variable during thestable arc phase (SP) whereas the arc voltage level is kept constant.The feed speed of the hot wires is continually adjusted to maintain theamperage level of the welding current at the set stable arc phasewelding current level and compensates for disturbances such as surfaceirregularities and variations in the welding process. This CA weldingprocess is easy to carry out and provides good welding results.

FIG. 6 b shows an alternative embodiment of the method according to theinvention. In this embodiment, the welding phase is preceded by a firstand a second start-up phase and the arc-stabilizing phase (AP) isdivided into three sub-phases: an initial sub-phase (IS) correspondingto the first start-up phase, a main sub-phase (MS) corresponding to thesecond start-up phase, and a final sub-phase (FS) covering the timeperiod between the end of the second start-up phase and the beginning ofthe stable arc phase (SP).

The purpose of this arrangement is to facilitate the generation of astable arc and to postpone the welding phase until a stable arc has beengenerated.

In addition to the stable arc welding current and any additional weldingparameter values related to the stable arc phase, the user also enters aplurality of sub-phase specific welding parameters. Examples of suitablewelding parameters are welding current, arc voltage and travel speed.Based on these values, the control unit determines a suitable hot wirefeed speed for each sub-phase.

The main purpose of the initial sub-phase (IS) is to facilitate thegeneration of a stable arc. This is achieved by temporarily removing thenegative effect of the cold wire on the arc-stabilization process. Thecold wire is kept still during the initial sub-phase (IS) whereas thehot wires are fed forward at the constant feed speed determined by thecontrol unit.

The main purpose of the main sub-phase (MS) is to introduce the coldwire into the welding process while ensuring that the arc remainsrelatively stable. Both the cold wire and the hot wires are now fedforward at constant feed speeds determined by the control unit. In thisembodiment, the hot wire feed speed is higher during the main sub-phase(MS) than during the initial sub-phase (IS). The cold wire feed speed isdetermined in dependence on the corresponding first hot wire feed speed.

The hot wire feed speed applied during the final sub-phase (FS) ishigher than the hot wire feed speed applied during the main sub-phase(MS), and thus closer to the expected stable arc phase hot wire feedspeed. Thus, a smooth transition from the final sub-phase (FS) to thestable arc phase (SP) is ensured. The cold wire feed speed follows thefirst hot wire feed speed and is also increased at the beginning of thefinal sub-phase (FS). In alternative embodiments, the main sub-phase(MS) may be extended to the end of the arc-stabilizing phase (AP).

The method shown schematically in FIG. 6 a will now be described more indetail with reference to FIG. 7 a. The solid line (A) shows the actualhot wire feed speed during each phase, the solid line (B) shows theactual cold wire feed speed during each phase, and the dotted line (C)shows the expected hot wire feed of the stable arc phase (SP).

FIG. 7 a shows that the hot wire feed speed during the ignition phase(IP, t₀-t₁) is set to a value of about 25% of the expected stable arcphase hot wire feed speed (C).

As mentioned, the expected stable arc phase hot wire feed speed (C) isdetermined by the control unit based on one or more welding parametervalues set by the user before the welding process begun. The actualfirst hot wire feed speed (A) at the beginning (t₃) of the stable arcphase (SP) is in this embodiment equal to the expected stable arc phasehot wire feed speed (C). Note that this is not always the case. Theactual first hot wire feed speed (A) may differ from the expected stablearc phase hot wire feed speed (C), for example as a consequence ofencountered disturbances.

An arc detection means detects an arc and sends a signal to the controlunit (t₁). The control unit instructs the hot wire feeding means toincrease the hot wire feed speed (A) to a value of about 90% of theexpected stable arc phase hot wire feed speed (C). The new hot wire feedspeed (A) remains constant throughout the initial sub-phase (IS, t₁-t₂)and the main sub-phase (MS, t₂-t₃). The cold wire is kept still duringthe initial sub-phase (IS, t₁-t₂) and assumes a constant cold wire feedspeed (B) during the main sub-phase (MS, t₂-t₃). The cold wire feedspeed (B) during the main sub-phase (MS, t₂-t₃) is about 70% of thefirst hot wire feed speed (A) during the main sub-phase (MS, t₂-t₃).

The arc-stabilization phase (AP, t₁-t₃) lasts 2.5 seconds and isimmediately followed (t₃) by the stable arc phase (SP). As thearc-stabilization process enters the stable arc phase (SP), the controlunit instructs the hot wire feeding means to apply a feed speed to thefirst and second hot wires corresponding to the stable arc phase weldingcurrent set by the user of the welding apparatus. Thereafter, theamperage level of the welding current is continuously measured and themeasured values are filtered in the control unit. The welding currentamperage level may vary during the stable arc phase (SP) as aconsequence of disturbances and the control unit is programmed torestore the amperage level to the set value (the stable arc phasewelding current) through adjustment of the hot wire feed speed (i.e. thecontrol unit is adapted to maintain the amperage level at said set levelthrough regulation of the hot wire feed speed). For example, at t₅, thecontrol unit registers an increase of the welding current amperage leveland thus instructs the hot wire feeding means to lower the hot wire feedspeed (A) in order to restore the amperage level to its previous, lowervalue.

The first hot wire feed speed is continuously measured during thewelding process and the measured values are filtered by the controlunit. For each filtered value, the control unit determines acorresponding feed speed target value for the cold wire and the controlunit instructs the cold wire feeding means to adjust the cold wire feedspeed (B) to said target value. Consequently, at t₂ and t₃-t₄, the coldwire feed speed (B) is increased and at t₅ the cold wire feed speed (B)is reduced.

Note that the increase of the cold wire feed speed (B) as thearc-stabilization process enters (t₃) the stable arc phase (SP) isexecuted at a relatively slow rate (in comparison to the almostinstantaneous increase of the first hot wire feed speed (A)). The coldwire feed speed (B) reaches its new target value at t₄. This delayensures that the arc is stabilized at the new and higher hot wire feedspeed (A) level before the cold wire feed speed (B) reaches its new andhigher target value. Note also that a reduction of the cold wire feedspeed (B) at t₅ is carried almost instantaneously, to avoid a situationwherein the cold wire has a feed speed (B) too high in relation to thehot wire feed speed (A) and strikes through the weld puddle.

The easiest way to determine whether the cold wire feed speed (B) is tobe increased or reduced is to store the last measured and filtered firsthot wire feed speed (A) value in the control unit and compare it to thenext measured and filtered first hot wire feed speed (A) value.Similarly, the last determined cold wire feed speed (B) value can bestored in the control unit and compared to the next determined cold wirefeed speed value (B).

At the transition (t₃) from the arc stabilizing phase (AP) to the stablearc phase (SP), the control unit compares the first measured andfiltered hot or cold wire feed speed value to the corresponding constantfeed speed value kept during the main sub-phase (t₂-t₃) of thearc-stabilizing phase (AP), to determine whether the adjustment of thecold wire feed speed (B) should be carried out instantaneously (areduction) or with a delay (an increase).

The method shown schematically in FIG. 6 b is shown more in detail inFIG. 7 b.

The arc-stabilizing phase (AP) in FIG. 7 b is divided into threesub-phases: an initial sub-phase (IS, t₁-t₂), a main sub-phase (MS,t₂-t₃) and a final sub-phase (FS, t₃-t₄).

The first hot wire feed speed during the arc ignition phase (IP, t₀-t₁)is about 25% of the expected stable arc phase hot wire feed speed (C).The hot wire feed speed (A) is increased to about 50% of the expectedstable arc phase hot wire feed speed (C) once an arc has been detected(t₁) and the arc-stabilization process begins with the initial sub-phase(IS, t₁-t₂). The cold wire remains stationary to optimize the chances ofgenerating a stable arc during said initial sub-phase (IS, t₁-t₂). Thehot wire feed speed (A) is increased again at the beginning (t₂) of themain sub-phase (MS, t₂-t₃) to about 75% of the expected stable arc phasehot wire feed speed (C). Simultaneously, the cold wire is being fedforward at a feed speed (B) of about 50% of the first hot wire feedspeed (A). Thus, the arc is allowed to stabilize in the presence of thecold wire before the stable arc phase (SP) begins. Once the mainsub-phase (MS, t₂-t₃) has come to an end and the final sub-phase (FS,t₃-t₄) begins, the hot wire feed speed (A) is increased again to about90% of the expected stable arc phase hot wire feed speed (C) and thecold wire feed speed (B) is increased to 70% of the first hot wire feedspeed (A). Finally, at the beginning of the stable arc phase (SP), thefirst hot wire feed speed (A) is released and the cold wire feed speed(B) is continuously adapted to the first hot wire feed speed, asdescribed above with reference to FIG. 7 a.

In this embodiment, the hot wire feed speed (A) is increased every timethe arc-stabilization process enters a new sub-phase (the same is truefor the cold wire feed speed with exception for the transition from thearc ignition phase (IP) to the initial sub-phase (IS)). The hot wire andcold wire feed speeds remain constant during each sub-phase. The hotwire feed speed (A) levels are determined by the control unit based onthe welding parameter values set by the user before the welding processbegan and the cold wire feed speed (B) values are determined independence on the corresponding first hot wire feed speed (A) values.During the stable arc-phase (SP), target values for cold wire feed speed(B) is determined by means of the control unit based on measured firsthot wire feed speed (A) values.

The scope of protection provided by the claims is not limited to theabove described embodiments. Embodiments and features can be combined inmany ways without falling outside the scope of protection.

For example, a change in welding conditions during the arc-stabilizingphase may cause the hot wire feed speed to be reduced when thearc-stabilization process enters the stable arc phase. There are alsoembodiments wherein the stable arc phase welding current to bemaintained during the stable arc phase is so low that it may beadvantageous to apply a hot wire feed speed higher than the expectedstable arc phase hot wire feed speed during the arc-stabilizing phase.The methods described above with reference to FIGS. 6 a,b ; 7 a,b can beapplied on a welding apparatus comprising any number of hot wires andmore than one cold wire. A hot wire may have the same feed speed in twoor more sub-phases, and a cold wire may have the same feed speed in twoor more sub-phases. The arc-stabilizing phase in FIG. 7 b may comprisemore than three sub-phases. It is also possible to remove the finalsub-phase and extend the main sub-phase to the end of thearc-stabilizing phase. In embodiments wherein an arc-stabilizing phaseis to be terminated before the end of the last start-up phase, thearc-stabilizing phase may be extended to the end of the last start-upphase.

The invention claimed is:
 1. A method for starting a submerged arcwelding process comprising: initiating, via a control unit, an arcignition phase (IP) of a starting process comprising igniting an arc;subsequent to the arc ignition phase (IP), executing, via the controlunit, an arc-stabilizing phase (AP) of the starting process comprisingat least an initial sub-phase (IS) and a main sub-phase (MS) that issubsequent to the initial sub-phase (IS), wherein: said initialsub-phase (IS) comprises feeding at least one hot wire towards a workpiece at a first constant feed speed, and said main sub-phase (MS)comprises feeding said at least one hot wire towards the work piece at asecond constant feed speed and feeding at least one cold wire towardsthe work piece at a third constant feed speed; and subsequent to thearc-stabilizing phase (AP), executing, via the control unit, a stablearc phase (SP) of the starting process comprising continuously adjustinga fourth feed speed of the at least one hot wire in dependence on atleast a welding current transferred through said at least one hot wireand continuously adjusting a fifth feed speed of the at least one coldwire in dependence on the first constant feed speed, the second constantfeed speed, or the fourth feed speed of the at least one hot wire. 2.The method according to claim 1, wherein a sixth feed speed of the atleast one cold wire in a direction towards said work piece during theinitial sub-phase (IS) remains equal to or lower than 9 cm/min.
 3. Themethod according to claim 1, wherein the at least one cold wire remainsstationary during the initial sub-phase (IS).
 4. The method according toclaim 1, wherein the first constant feed speed of the at least one hotwire is equal to the second constant feed speed of the at least one hotwire so that the at least one hot wire is fed at a constant speedthroughout the arc-stabilizing phase (AP).
 5. The method according toclaim 1, wherein the first constant feed speed of the at least one hotwire varies from the second constant feed speed.
 6. The method accordingto claim 1, wherein: at least one of the first constant feed speed ofsaid at least one hot wire and the second constant feed speed of the atleast one hot wire is in a range of 0-200% of an expected speed of thefourth feed speed of said at least one hot wire during the stable arcphase (SP) based on a welding current during the stable arc phase (SP);and the third constant feed speed of the at least one cold wire duringat least one of the initial sub-phase (IS) and the main sub-phase (MS)is in a range of 0-100% of said at least one of the first constant feedspeed of said at least one hot wire and the second constant feed speedof the at least one hot wire.
 7. The method according to claim 6,wherein at least one of the first constant feed speed of said at leastone hot wire and the second constant feed speed of the at least one hotwire is in a range 0-100% of the expected speed of the fourth feed speedof said at least one hot wire during the stable arc phase (SP).
 8. Themethod according to claim 7, wherein at least one of the first constantfeed speed of said at least one hot wire and the second constant feedspeed of the at least one hot wire is in a range 80-95% of the expectedspeed of the fourth feed speed of said at least one hot wire during thestable arc phase (SP).
 9. The method according to claim 6, wherein thethird constant feed speed of the at least one cold wire is in a range of70-90% of the second constant feed speed of the at least one hot wireduring the main sub-phase (MS).
 10. The method according to claim 1,wherein said arc-stabilizing phase (AP) has a predetermined length in arange of 1-6 seconds.
 11. The method according to claim 10, wherein thepredetermined length in a range of 1-3 seconds.
 12. The method accordingto claim 1, wherein the initial sub-phase (IS) has a predeterminedlength in a range of 0.5-5 seconds.
 13. The method according to claim 1,comprising feeding more than one hot wire towards the work piece.